Use when two sides of an argument each look obviously right to themselves and obviously wrong to the other — check whether they share a definition of 'what would count as proof' at all. If not, you're in an incommensurability situation, and arguing facts is pointless until you argue the yardstick. Use to spot when a new strategy can't be evaluated by the old one's metrics.

Kuhn's hardest claim: after a revolution, the new tradition and the old one don't just disagree — they can't be compared point-by-point, because what even counts as a valid problem, a valid solution, or a valid standard has changed. There is no neutral observation language both sides accept, so 'who's right' can't be settled with a shared ruler.